Automatic Gain Controls are widely used to automatically change the gain or amplification of an input signal to provide an output signal having a substanially constant power level. For example the power level of an input signal might vary within a range of 40 db while the power level of the output signal of a typical Automatic Gain Control might vary within a range of only .+-. 3 db.
These Automatic Gain Controls typically include an amplifier followed by an attenuator which has a plurality of taps adjacent pairs of which are separated, for example, by a signal attenuation of 6 db. If the input signal varies from zero to -40 db, the amplifier will typically have a constant amplification of +40 db. This will provide at the output of the amplifier a signal having a power level which varies between zero and +40 db.
By monitoring either the power or the RMS value of the input signal, the particular tap in the attenuator is chosen which most closely approximates the 0 db level. In the worst case, this zero db level may be centered between two taps which are separated, for example, by a power level of 6 db. Under these circumstances, chosing either of the taps would provide the signal at the output of the Automatic Gain Control with a power level which is 3 db off the desired 0 db level.
In this manner, the Automatic Gain Controls of the prior art have reduced power level variation from a relatively high level such as -40 db to a relatively low level such as .+-. 3 db. Even with this significant reduction, the .+-. 3 db power level variations are undesirable and particularly adverse in digital applications.
In digital systems, the Automatic Gain Control is typically followed by an analog-to-digital converter. This converter samples the analog signal at the output of the Automatic Gain Control and provides a plurality of digital symbols or words each expressing the amplitude of the associated sample. The digital words or symbols are processed in the remaining portions of the digital system to detect the data transmitted. Each of the digital words includes digits or bits the number of which determine the maximum word length.
One problem with a significant power level variation in the output signal from the Automatic Gain Control is that, under certain conditions, the digital samples are grossly inaccurate. For example in some systems the maximum word length of the converter is used to express the peak level of a 0 db signal from the Automatic Gain Control. Theoretically this is desirable to increase the accuracy with which the amplitude of the samples are expressed. However, in the worst case where the power level of the signal from the Automatic Gain Control varies, for example, + 3 db, the converter attempts to express the resulting peak sample in a digital word having a length greater than the maximum word length. This undesirable result, which is commonly referred to as overflow or saturation produces grossly inaccurate symbols.
One method of reducing the probability of overflow or saturation is to reduce the amplification of the input signal so that extreme fluctuations in the power level do not exceed the maximum word length of the digital symbols. In this manner a particular portion of the word length is reserved to accommodate this undesirable power level fluctuation. This reduces the probability that a high noise signal would produce a digital symbol greater than that which could be expressed by the available word length.
By reserving a portion of the word length to reduce overflow, however, the remaining portion of the word length devoted to expressing the actual signal level is reduced. As a consequence, fewer digital words are available to express the actual signal level. This in turn results in an undesirable increase in the quantization noise of the sampler.
To express the signal strength more accurately, the word length of the symbols has been increased. However, increases in the word length are particularly expensive to implement since all portions of the system have been updated to accommodate symbols with additional bits per word. Furthermore, the increased word length does not reduce the quantization noise which has remained a significant source of performance degradation in these digital systems.